1. Field of the Invention
The present invention relates to a method for manufacturing a cable.
In particular, the present invention relates to a method for manufacturing a cable which comprises the step of extruding a polymeric material to obtain at least one coating layer arranged in a radially outer position with respect to a cable transmissive element.
The invention further relates to an extrusion head for extruding a polymeric material and to an extrusion apparatus which comprises said extrusion head.
2. Description of the Related Art
Throughout the present description and the claims, the term “transmissive element” is used to indicate an electrical energy transmissive element, an optical signal transmissive element and/or an element which is suitable for carrying electrical energy and optical signals.
The term “electrical energy transmissive element” is used to indicate any element capable of transmitting electrical power, e.g. a metal conductor element.
The term “optical signal transmissive element” is used to indicate any transmissive element which comprises at least one optical fibre. This term identifies a single optical fibre as well as a plurality of optical fibres, optionally grouped together to form a bundle of optical fibres or arranged parallel to each other and coated with a common coating to form a ribbon of optical fibres.
The term “combined electro-optical transmissive element” is used to indicate any element capable of providing both electrical energy and optical signals in accordance with the abovementioned definitions (i.e. a combination of one or more electrical energy transmissive element and one or more optical signal transmissive element).
Generally, a cable comprises at least one elongated transmissive element (electrical, optical or electro-optical element) and at least one coating layer which is made of a polymeric material and which is arranged at a radially outer position with respect to said elongated transmissive element.
Throughout the present description and the following claims, the term “cable” is intended to include unipolar or multipolar cables of the electric type for transporting or distributing electrical power, or of the optical type comprising at least one optical fibre for telecommunications, or of the combined power telecommunications type.
The polymeric material of said coating layer is selected so as to provide the latter with semiconductive properties, electrical insulation or mechanical protection properties depending on the type of cable under consideration as well as on the position of said coating layer in the cable.
For instance, in case an electrical cable for transporting or distributing medium/high voltage electrical power is considered, said cable comprises: at least one metal conductor element; an inner semi-conductive coating arranged at a radially outer position with respect to said conductor element; an electrical insulation element surrounding said inner semi-conductive coating; an outer semi-conductive coating arranged at a radially outer position with respect to the electrical insulation element; a metallic screen arranged at a radially outer position with respect to said outer semi-conductive coating, and a polymeric outer sheath protecting the cable from the external environment. Generally, the term “medium voltage” is used to indicate a voltage comprised between about 1 kV and about 30 kV, while the term “high voltage” is used to indicate a voltage greater than about 30 kV.
In order to provide the elongated transmissive element with a polymeric coating layer, the latter is generally obtained by extrusion.
Generally, an extrusion apparatus comprises an extrusion head which includes: a male die; a female die, coaxially arranged with respect to the male die, and a distributor element for uniformly distributing the extruded material into a conveying channel which is provided between the male die and the female die. The male die is usually provided with an inner cavity coaxially extending with respect to a longitudinal axis of the extrusion head, said cavity being suitable for receiving the at least one elongated transmissive element advancing along a direction substantially parallel to said longitudinal axis. The polymeric material flowing into the conveying channel is thus deposited onto the transmissive element advancing through the extrusion apparatus.
In order to obtain a product with desired physical-chemical and geometrical characteristics, a manufacturing process requires to be carried out at a given stationary state (e.g. at a value of flow rate, pressure, temperature, line speed within a given range) which ensures that said characteristics can be achieved. The manufacturing process reaches said stationary state by passing through at least one transient state during which the process parameters (e.g. flow rate, line speed, pressure, temperature) are varied to obtain the predetermined working conditions at which the desired product characteristics can be achieved.
A transient state occurs, for instance, at the beginning of a manufacturing process as well as at the end thereof. Moreover, a transient state also occurs when a cable length greater than the available length of the elongated transmissive element is to be produced and the empty reel or reels—on which the elongated transmissive element(s) is (are) supported to be fed to the extrusion apparatus—needs to be changed. In this case, the line speed is reduced to allow the replacement of the supply reel of the transmissive element (e.g. the conductor).
In a cable manufacturing process, the extrusion speed (i.e. the speed line of the manufacturing process) is caused to vary during the transient states since the linear velocity of the elongated transmissive element passing through the extrusion head is increased or decreased departing from the stationary working conditions.
During manufacturing, the Applicant has noted that a variation of the extrusion speed can cause a relevant variation of the cable quality being produced and uniformity and homogeneity of the extruded layer as well as the cable electrical/mechanical performances can not be always guaranteed. In some circumstances, the quality of a cable portion produced during a transient state can be remarkably lower than the requested one, fact which causes said cable portion (whose length can be in the order of several hundred meters) to be discarded and thus the production costs to sensibly increase. Moreover, a variation of the extrusion speed also influences the time of permanency of the polymeric material in the extrusion head. This fact is particularly critical when polymeric materials that are sensitive to temperature (e.g. cross-linkable materials) are used. In fact, when long periods of permanency of said materials are caused to occur and stagnation zones thereof are formed in the extrusion head, scorching at high temperatures and/or clots at low temperatures of said materials can arise. Scorching and clots need to be avoided since they negatively influence uniformity and homogeneity of the extruded coating layers, and thus of the overall quality and performances of the manufactured cable. Moreover, said defects can cause the extrusion process to be stopped in order to allow the extrusion head to be cleaned from the clots and/or the scorched material formed on the walls of the extruder barrel and/or on the extruder screw(s).
The Applicant has further observed that a variation of the flow rate of the extruded polymeric material in connection with a variation of the extrusion speed causes a variation of pressure inside the extrusion head. For instance, an increase of the extrusion speed (e.g. during a process transient state) requires to increase the flow rate of the extruded polymeric material in order to maintain the desired geometry of the extruded layer, which causes an increase of the pressure inside the extrusion head. Since the pressure of the extrusion head is limited by the materials used for making the extrusion head as well as by the geometry thereof and, moreover, since an increase of said pressure corresponds to an increase of the temperature of the polymeric material which is being extruded, in order not to mechanically stress the extrusion head and not to scorch the polymeric material, the flow rate of the polymeric material needs to be kept below a predetermined maximum value, fact which inevitably limits the maximum extrusion speed value. On the contrary, a decrease of the extrusion speed (e.g. during a process transient state) requires to decrease the flow rate of the extruded polymeric material, which causes a decrease of the pressure inside the extrusion head. As a consequence, the time of permanency of the polymeric material in the extrusion head increases and, as mentioned above, scorching of the polymeric material as well as formation of stagnation zones can occur. Therefore, the flow rate of the polymeric material needs to be kept over a predetermined minimum value, fact which inevitably limits the ability of the process to follow the speed requirements of the manufacturing line.
It is known to manufacture a cable with a coating layer made of an expanded polymeric material, said layer being provided, for instance, in order to increase the cable resistance to accidental impacts which can occur on the cable during transport and/or lying thereof. For example, European Patent No. 981,821—in the name of the Applicant—discloses a cable which is provided with a layer of expanded polymeric material in order to confer to said cable a high resistance to accidental impacts, said layer of expanded polymeric material being preferably applied radially external to the cable core. This technical solution avoids the use of traditional metal armours, thereby reducing the cable weight as well as making the cable manufacturing process easier.
The Applicant has noted that, in case an expandable polymeric material is extruded to provide the cable with an expanded coating layer, an extrusion speed variation and associated flow rate variation—which can occur, for instance, during the process transient states—causes the expansion degree of the extruded polymeric material to vary during extrusion. In other words, the Applicant has noted that the expansion degree of the expanded polymeric coating which is produced during the process transient states may not be maintained at a desired constant value.
In detail, the Applicant has noted that the expansion degree of the expanded polymeric coating layer principally depends on the pressure drop which occurs at the exit of the extrusion head. Therefore, at a given amount of the expanding agent being used and at a given extruder thermal profile which has been chosen, an extrusion speed variation occurring during a transient state causes a variation of the expansion degree of the expanded polymeric coating layer. The variation of the expansion degree is believed to be due to the fact that, by setting a variation of the extrusion speed and a consequent variation of the flow rate of the polymeric material being extruded, a pressure variation in the polymeric material flowing through the extrusion head arises. As a consequence, the cable portion produced during the transient state is provided with an expanded coating layer whose expansion degree is not uniform along the length thereof.
Document U.S. Pat. No. 3,752,614 discloses an extrusion head for forming insulated wire which includes a fixed threaded hollow mandrel and a threaded hollow pin disposed internally of, and in mating engagement with, the mandrel for supporting a male die member in axial alignment with a female die member mounted within the head. The threaded portions of the mandrel and the pin are so engaged that rotation of the pin within the mandrel advances or retracts the male die member with respect to the female die member while maintaining the alignment therebetween. This device allows to compensate for changes in the extruded plastic material, insulation thickness, or in the pressure or temperature of the system while the extrusion head is in operation. The relative movement between the male die member and the female die member is effected by an operator by manual control.
Document U.S. Pat. No. 3,583,033 discloses a die for in-line extrusion of viscoelastic and viscous thermoplastic materials, comprising a conical male valve member which is advanced or retracted with respect to a conical seat to vary the degree of shear and back pressure to which the material is exposed in passing through the annular conical passageway. The movement of the conical male valve member is achieved by rotating a ring nut and is manually effected and controlled by an operator.
Document GB-2,060,473 discloses a head for extruding tubes for blow moulding, including a mandrel supported by one part, the other part comprising at least one conical wall portion which, with a corresponding mandrel wall portion, forms a conical flow space section whose throughflow cross-section can be varied by the relative displacement of the two telescopically engaging parts. The relative displacement is manually effected by the operator by means of an adjusting screw associated with one part of the mandrel and engaged with a suitable screwthread formed on the other part of the mandrel. It is described that various remote controlled servo devices could conceivably be used in place of the adjusting screw.
Document U.S. Pat. No. 3,402,427 discloses a crosshead die body apparatus including a shaping die for extruding and shaping thermoplastic material comprising polyvinylidene fluoride resin, wherein the crosshead die body has at least two externally adjustable internally axially positioned frusto-conically shaped valving means and at least one annular orifice portion of fixed uniform annular width and of substantially fixed but adjustably variable length located axially between said valving means, whereby the pressure drop and shearing stress between the extruder outlet and the shaping die may be progressively and precisely controlled. During start-up of the coating process, the surface and body characteristics of the extrudate are observed by the operator of the machine and are modified by manipulation of the valving means until the optimum extrudate characteristic are obtained. Then, by continued observation and manipulation of the valving means, either by manual or automatic control, the optimum characteristics can be maintained by the operator throughout the extruding and shaping operation without reaching or exceeding the yield point of the resin.